1. Field of the Invention
The present invention relates to a device for heating chemical tanks.
2. Description of the Prior Art
In microelectronics manufacturing, one technique for etching silicon slices involves immersing them in a solution. The solution can be, for example, a heated mixture of dilute hydrofluoric acid and ammonium fluoride in the proportion of one to seven by volume. In a typical etch process, twenty-four liters of solution must be maintained within a temperature range of 28.degree. C. to 38.degree. C., with a tolerance of plus or minus 1.degree. C., according to the type of process to which the silicon is subjected.
According to the prior art, heating of the mixture takes place through the use of Teflon coated electrical resistance heaters, or of corrugated coils made of Teflon, both immersed in the solution. The coils use hot water, generated by automatic external heaters, as the heating element. A temperature sensor immersed in the solution controls the external heater so as to maintain the desired temperature of the solution.
The application of the systems described above has some drawbacks.
Insofar as the heating systems based on Teflon coated resistances are concerned, it is known that the protective coating deteriorates with use and handling due to cleaning. Microfractures are created in the Teflon coating, which allow the etching solution to directly contact the metal of the resistance. As a consequence, the solution become contaminated with metal ions.
Insofar as the heating systems based on hot water coils are concerned, microfractures are created in the corrugated Teflon coil as a result of use and handling due to cleaning. Heating water infiltrates into the etching solution through the microfractures, contaminating it with ions of various kinds present in the heating water itself.
The contaminating ions introduced into the solution by the heating systems described above deposit themselves on the silicon slices contained in the etching tank. This damages the integrity of the slices, and increases the number of rejects. The increase in rejects can become a quite significant factor.